Vehicles incorporating tandem drive axles benefit in many ways over vehicles having a single driven axle. Inter-axle differentials in such vehicles may be configured to distribute torque proportionately or disproportionately between the axles. Additionally, shift mechanisms may be provided to such vehicles to permit the disengagement of one of the driven axles or to transition from single axle operation to tandem axle operation, among other benefits. However, such versatility typically requires the incorporation of additional drive train components into the vehicle at added expense and weight. Such added weight results in a decreased fuel efficiency of the vehicle.
Components of the tandem drive axle may also be selected based on a gear reduction ratio present in an axle. Axle ratios may be of a two-speed configuration to permit the vehicle to operate in a low speed and high torque manner or in a high speed and low torque manner. It is preferred to drive both axles when the low speed and high torque manner of operation is desired (to distribute the higher torque amongst a greater number of wheels) and it is advantageous to operate a single axle when the high speed and low torque manner of operation is desired (to decrease windage and frictional losses when torque distribution is of lower concern). However, incorporation of both the two-speed configuration, an axle disconnect function, and the inter axle differential may be prohibitive with respect to cost and weight. Such added weight, windage losses, and frictional losses result in a decreased fuel efficiency of the vehicle.
Additionally, when components are provided to permit the disengagement of one of the driven axles or to transition from the vehicle from single axle operation to tandem axle operation, a complexity of the tandem drive axle is increased. Typically, such operations may require manual engagement by an operator when specific conditions are present or may require the operator to stop the vehicle to engage specific components. Such features, while providing additional functionality to the vehicle, may not be properly implemented by the operator or may not be used at all by the operator. When such features are not properly implemented, a shift from one operating state to another may result in damage to the tandem drive axle or a rough shift, either of which typically result in a dissatisfaction of the operator.
It would be advantageous to develop a method of shifting a tandem drive axle system from a low speed and high torque tandem axle manner of operation to a high speed and low torque single axle manner of operation that reduces windage and frictional losses and facilitates improved shifting from one operating state to another without excessively increasing a cost and a weight of the tandem drive axle.